A conventional strain sensor disclosed in Japanese Patent Laid-Open Application No. 2000-180255 is structured of one glass layer and one overcoat glass layer. In the following, a conventional strain sensor is described referring to drawings. FIG. 7 shows plan view of a conventional strain sensor.
Metal substrate 1 is provided at one end with first fixing hole 20, second fixing hole 21 at the other end, and detection hole 22 in the approximate middle part. On the upper surface of metal substrate 1, glass layer 2 is formed, and four strain detecting resistors 6 are provided thereon. Strain detecting resistors 6 are electrically connected by wiring 11 and second electrode 5 to form a bridge circuit. Strain detecting resistors 6 and second electrode 5 are protected by overcoat glass layers 7a, 7b. 
FIG. 8 shows cross sectional view of a conventional strain sensor, sectioned along the line A–A′ of FIG. 7. Since glass layers 2a, 2b and 2c are made of same lead borosilicate system glass material, these layers are integrated into a single layer after being sintered. So, individual glass layers can not be distinguished severally in the layer.
In FIG. 8, these glass layers 2a, 2b and 2c are illustrated severally by providing a broken line between the layers for the sake of easy understanding. Round voids 9 are scattered at random within the glass layers after sintering.
Reason why the glass layer in FIG. 8 is illustrated in three layers is that Japanese Patent Laid-Open Application No. H09-243472 teaches formation of a multi-layered insulation layer by printing a 20 μm thick glass paste of lead boro-silicate system glass material and sintering it for three times (printing-sintering is repeated for three times), instead of using enamel or crystalline glass.
A method of assembling a conventional strain sensor is described in the following with reference to FIG. 7.
On the upper surface of already-prepared metal substrate 1, a glass paste is screen-printed and sintered at an approximate temperature of 850° C. for forming glass layer 2 on the upper surface of metal substrate 1. On the upper surface of glass layer 2, a conductive paste of Ag and Pt is screen-printed and sintered at an approximate temperature of 850° C. for forming wiring 11 and second electrode 5 on the upper surface of glass layer 2. And then, a Ru system resistance paste is printed covering part of glass layer 2 and second electrode 5, and sintered at an approximate temperature of 850° C. for forming strain detecting resistor 6. Finally, a glass paste is screen-printed covering glass layer-2, wiring 11, strain detecting resistor 6, and sintered for forming overcoat glass layers 7a, 7b. 
If a window is provided in advance in the pattern of overcoat glass layer, a chip component or semiconductor device can be mounted and connected with wiring 11 which is exposed through the window. Operation of the above-configured conventional strain sensor is described below. Metal substrate 1 is fixed, at first fixing hole 20 and second fixing hole 21, on a fixed member (not shown) by means of bolt (not shown) and nut (not shown), and then a detection member (not shown) is fixed to detection hole 22. When an external force F is given from the above on the detection member (not shown), a deformation is caused on metal substrate 1.
As a result, strain detecting resistors 6 disposed on the upper surface of metal substrate 1 receive a compressive stress or a tensile stress, and the resistance value in each of respective strain detecting resistors 6 changes. Strain detecting resistors 6 are connected by wiring 11 to form a bridge circuit, and an external force F exerted on detection member (not shown) is detected in the form of differential voltage detected at the bridge. FIG. 4A shows relationship between the number of glass layers and the insulation resistance in conventional strain sensor as shown in FIG. 7, FIG. 8. FIG. 4A shows that the insulation resistance is in the level of ninth to eleventh power of 10, when the number of glass layers is as many as 3–4 layers. However, when the number of glass layers decreased to 2 layers, the insulation resistance decreased to the level of sixth to tenth power of 10. When the glass layer count decreased to 1, the insulation resistance decreased further down. According to result of measurement conducted on 1-layer glass layers, the insulation resistance was mostly lower than 1 Ω, or a state of short circuit; only a limited number of samples showed several hundreds K Ω. In the graph of FIG. 4A, the insulation resistance of 1-layer glass is shown to have the fifth power of 10, for the sake of simplification. As described in the above, the insulation resistance decreases sharply when the glass layer counts go lower (or, the glass layer thickness goes thinner), among those having conventional structure. The insulation resistance value also disperses wide, which generates a problem in the products reliability.
In practice, a strain sensor needs an insulation resistance that is in the ninth power of 10, or higher; which means that in the conventional structure three or more glass layers are needed.
This leads to a higher cost of finished products.